The present invention relates to a solid-state image sensing device and, more particularly, to a technique for compensating a picture element defect due to a crystal defect in a solid-state image sensing device.
As is well known, a solid-state image sensing device is made up of a number of photosensing cells arrayed on a semiconductor substrate in a matrix fashion. The image sensing device, because of many advantageous features, has gradually superceded the conventional image sensing device and has found a variety of applications. Nevertheless, the image sensing device, such as a charge-coupled device (CCD), still involves problems to be solved. One of the problems is a low production yield of CCDs. In manufacturing the solid-state image sensing device or CCD, it is very difficult to form, with good precision and high uniformity, a semiconductor crystal over a fixed area. If a chip cut from a silicon wafer has even one local crystal defect, the thermal noise carriers are generated at the defective portion. The generated noise carriers cause a dark current of the CCD to extraordinarily increase. As a result, in an image signal output from the CCD, the signal derived from the defective portion or the defective picture element also rises in level with the increase of the dark current. The level rise of the defective picture element signal gives rise to white noise in a reproduced picture. This type of noise greatly deteriorates the quality of the reproduced picture.
For the above reasons, in an inspection stage of the CCD manufacturing, if even a single defective picture element is found in a CCD chip, which contains several hundreds of thousands of picture elements, such a CCD chip is scrapped as a defective chip. This fact greatly contributes to deterioration of the manufacturing yield of CCDs. The deterioration of the manufacturing yield results in a rise in the cost to manufacture CCDs. The CCD chip has a fairly high integration density of cells, which is comparable with that of LSI. Therefore, a good technique is required for stably manufacturing a perfect substrate, free from the crystal defect, and hence CCD chips containing no defective photosensing cells. In this respect, to eliminate the crystal defect is the most primitive, but the most ideal approach. However, this approach inevitably encounters a limit in improving the production yield.
Many other approaches than the above have been proposed in which compensation for the cell defect or crystal defect is applied for a CCD chip containing the crystal defect or the cell defect. At the present stage of this technology, however, there has been proposed none of successful approaches satisfactorily reducing the crystal defects.
In one of the CCD crystal correcting approaches, as is disclosed in Japanese Patent Disclosure (KOKAI) No. 55-32270, an external circuit is provided exclusively for a CCD chip. Image signals output from the CCD chip are subjected to a electrical correction, thereby removing the problem of a picture element defect. Let us assume that, of the picture elments, the n-th picture element is defective, and that the crystal defective noise of an extrodinary high level due to the dark current increase is contained in the signal component derived from the defective picture element. In this approach, a picture element signal component of a level exceeding a predetermined level is electrically detected and is replaced by another normal picture element signal component, for example, the (n-1)th picture element component in a circuit processing manner. Such a correction approach by the digital signal still involves the following problems.
(1) It is impossible to improve the image signal output from the CCD chip.
(2) The external circuit is required in addition to the CCD chip, resulting in complexity in the hardware configuration.
(3) All of the picture element signal components of higher levels than the reference level are unconditionally replaced by another signal level.
Therefore, to secure a satisfactory S/N ratio, it is very difficult to appropriately set the reference level.